feat: configuration backup management with manual and auto backup

Add backup/restore functionality for the SQLite database. Users can
trigger manual backups, configure automatic backups on an interval
with retention policies, list/download/delete backups, and restore
from any backup.

- Backup engine using VACUUM INTO (safe with WAL mode)
- Backup metadata tracked in DB, files stored in DATA_DIR/backups/
- Settings: backup_enabled, backup_interval_hours, backup_retention_count
- API: POST/GET/DELETE /api/backups, download, restore endpoints
- Autobackup via cron scheduler with configurable interval
- Retention: prune on startup, after each backup (manual and auto)
- Orphan cleanup: removes backup files without metadata on startup
- Restore: replaces DB and triggers graceful server shutdown
- Settings UI: /settings/backup with toggle, interval, retention config
- Backup list with download, delete, restore actions
- i18n: English and Russian translations

internal/api/settings.go

@@ -30,10 +30,13 @@ type settingsRequest struct {
 	SSLCertificateID   *int    `json:"ssl_certificate_id,omitempty"`
 	StaleThresholdDays *int    `json:"stale_threshold_days,omitempty"`
 	AllowedVolumePaths *string `json:"allowed_volume_paths,omitempty"`
-	WildcardDNS        *bool   `json:"wildcard_dns,omitempty"`
-	DNSProvider        *string `json:"dns_provider,omitempty"`
-	CloudflareAPIToken string  `json:"cloudflare_api_token"`
-	CloudflareZoneID   *string `json:"cloudflare_zone_id,omitempty"`
+	WildcardDNS          *bool   `json:"wildcard_dns,omitempty"`
+	DNSProvider          *string `json:"dns_provider,omitempty"`
+	CloudflareAPIToken   string  `json:"cloudflare_api_token"`
+	CloudflareZoneID     *string `json:"cloudflare_zone_id,omitempty"`
+	BackupEnabled        *bool   `json:"backup_enabled,omitempty"`
+	BackupIntervalHours  *int    `json:"backup_interval_hours,omitempty"`
+	BackupRetentionCount *int    `json:"backup_retention_count,omitempty"`
 }
 
 // getSettings handles GET /api/settings.
@@ -62,6 +65,9 @@ func (s *Server) getSettings(w http.ResponseWriter, r *http.Request) {
 		"dns_provider":              settings.DNSProvider,
 		"has_cloudflare_api_token":  settings.CloudflareAPIToken != "",
 		"cloudflare_zone_id":        settings.CloudflareZoneID,
+		"backup_enabled":            settings.BackupEnabled,
+		"backup_interval_hours":     settings.BackupIntervalHours,
+		"backup_retention_count":    settings.BackupRetentionCount,
 		"updated_at":                settings.UpdatedAt,
 	})
 }
@@ -160,6 +166,25 @@ func (s *Server) updateSettings(w http.ResponseWriter, r *http.Request) {
 		updated.CloudflareZoneID = *req.CloudflareZoneID
 	}
 
+	// Backup settings.
+	if req.BackupEnabled != nil {
+		updated.BackupEnabled = *req.BackupEnabled
+	}
+	if req.BackupIntervalHours != nil {
+		if *req.BackupIntervalHours < 1 {
+			respondError(w, http.StatusBadRequest, "backup_interval_hours must be at least 1")
+			return
+		}
+		updated.BackupIntervalHours = *req.BackupIntervalHours
+	}
+	if req.BackupRetentionCount != nil {
+		if *req.BackupRetentionCount < 1 {
+			respondError(w, http.StatusBadRequest, "backup_retention_count must be at least 1")
+			return
+		}
+		updated.BackupRetentionCount = *req.BackupRetentionCount
+	}
+
 	if err := s.store.UpdateSettings(updated); err != nil {
 		respondError(w, http.StatusInternalServerError, "failed to update settings: "+err.Error())
 		return